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Provide a simple way of mounting almost the entire installed operating system read-only, atomically snapshot it, or share it between multiple hosts to save maintenance and space. Instead of spreading RPM package content all over the place in the filesystem, and artificially separate /bin from /usr/bin and /lib from /usr/lib, move all content to /usr and provide only symlinks in the root filesystem.

/usr on its own filesystem provides a lot of valuable options in custom setups. For historic reasons, we split-off more and more tools from /usr and put them in /. But, advanced features in today's systems can not really bootup with an empty /usr anymore. More and more fails in subtle ways in such setups.

Instead of moving more tools to /, we today already require /usr to be mounted from inside the initramfs, to be available before the real 'init' starts. The split of the root filesystem and /usr serves no purpose in Linux anymore and only complicates or prevents simple and more flexible setups.

There is no way to reliably bring up a modern system with an empty /usr, there are two alternatives to fix it: copy /usr back to the rootfs or use an initramfs which can hide the split-off from the system.

Historically /bin, /sbin, /lib had the purpose to contain the utilities to mount /usr. This role can now be taken by the initramfs. Because the initramfs knows, where to find the root partition (which includes /etc), it can parse /etc/fstab and other configuration files and mount /usr before it finally switches the root partition and executes /usr/bin/init. From this point on init mounts the remaining partitions in /etc/fstab and the system starts as usual.

The long-term plan is to clean up the mess and confusion the current split of / vs. /usr has created. All tools will move back to /usr where they belong, and the rootfs will only contain compat-symlinks into /usr. Almost the entire system installed by packages will reside in /usr. This will split all non-host specific data to /usr. /usr can then be seen as the Unix System Resources partition (/System), which defines the base operating system (e.g. F18 or RHEL-7).

This new /usr could be mounted read-only by default, while the rootfs is read-write and contains only empty mount points, compat-symlinks to /usr and the host-specific data like /etc, /root, /srv. Compared to today's setups, the rootfs will be very small. The new /usr could also easily be shared read-only across several systems, and it would contain almost the entire system. Such setups are more efficient, can optionally provide a lot more security, are more flexible, provide more sane options for custom setups, and are much simpler to setup and maintain.

This leaves us with the following well-defined directories, which compose the base of the system:

Simpler and cleaner overall file system layout, with full compatibility.

Clear separation of operating system and host specific resources.

Improve compatibility with other Unixes/linux, no confusion about tools install locations, no $PATH fiddling, all possible paths to a binary will always work. All binaries will be available on both /usr and / thus minimizing compatibility problems.

Improve compatibility with build systems such as GNU autotools who never have been aware of the /usr split in the first place

Minimize difference to other Unixes, such as Solaris, which already did the same move

Isolate the vendor-supplied mostly read-only operating system resources from the rest, thus allow snapshotting of the OS, and easy lightweight container OS duplication

[edit] What is currently broken with having /usr as a separate partition?

[edit] I don’t have /usr as a separate partition. What changes for me?

Nothing changes in functionality. All the old paths are reachable, because there a compat symlinks in place, which will not go away (at least not in the near future). All your scripts and binaries should work, like they did before. For the upgrade process to work, you will find /sbin, /bin, /lib and /lib64 mostly containing symbolic links. As soon, as these directories only contain symbolic links, the whole directory is replaced by only one symbolic link. These three or four toplevel symbolic links will stay there as long as the linux elf loader ABI is defined with “/lib/ld-linux.so.2” or their architecture specific counterpart like “/lib64/ld-linux-x86-64.so.2”, and as long as scripts use “#!/bin/sh”.

[edit] Why don’t you fix the /usr situation by putting all the relevant binaries in /bin /sbin /lib and /lib64?

Then you would share the toplevel directory hierarchy among all hosts. Hosts would need to mount /etc and /var for host-only versions. Especially /etc/fstab is not accessible, without adding information to the initramfs on how to mount it. For every host-only additional top level directory like /opt and /srv, you would have to have a mountpoint.

[edit] So, why don’t you just mount /usr from the initramfs and leave the files where they are?

Ok, so imagine you have a /usr mounted from a network location and you want to update a package. So maybe you mount the master copy of /usr on your master machine and update /usr with your package manager. Then you provide a new copy of the master /usr to the other machines, when they reboot. They all have the new updated /usr now. But what about /sbin /bin /lib and /lib64? They still have the old binaries. No glibc security update for them. So, every machine has to update these directories via rsync or such (rpm will not work with a readonly /usr). This doubles the maintenance to keep both parts of the system in sync.

[edit] You are doing it wrong! /bin and /sbin are there to rescue a broken /usr!

The most critical filesystem is /boot, because the kernel lives there. So the purpose of having /bin and /sbin for /usr repairing relied on _two_ working filesystems ( / and /boot). If either of them was broken, you were not able to rescue /usr. The role of the rescue system can easily be fulfilled by a rescue initramfs. So having the rescue initramfs in /boot, which contains the fsck utils, is in the same danger of becoming corrupted as the kernel. Now you only have to pull out your rescue CD, if /boot is corrupted and not if / is corrupted.

[edit] Then, let’s share /bin /sbin /lib /lib64 and /usr and mount them all from the initramfs!

Now, you get a feeling, that moving everything to /usr might make things easier....

[edit] Why don’t you move all /usr contents to / and forget about /usr?

Because this introduces a lot of new toplevel directories, which all have to be mount points then to be shared across other hosts.

[edit] Ok, but what about a root filesystem on the network and mounting local filesystems only?

Then you would share the toplevel directory hierarchy among all hosts. Hosts would need to mount /etc and /var for host-only versions. Especially /etc/fstab is not accessible, without adding information to the initramfs on how to mount it. For every host-only additional top level directory like /opt and /srv, you would have to have a mountpoint.